Customer Wants Burnished Finish
A customer has asked us to provide a shiny, burnished finish to powdered metal parts.
Q. We use vibratory finishing to deburr powdered metal parts. Most of the time we are using 5/8-inch Tricyl ceramic media. (Tricyl is the name of a media shape that is also known by other names: double-cut cylinder, V-cut cylinder, and WEJ—a trademark of Markee.)
Our time cycles average about 20 minutes in either our bowl-shaped machines or in our tub-style machine. All of this is done to our satisfaction. One customer, however, has asked us to provide a shiny, burnished finish. His other supplier uses steel burnishing media, so we obtained enough ¼-inch carbon steel ball-cones for our 5-cu-ft bowl, the only one of our machines capable of using steel media. The finish is much brighter than we have ever obtained, but the customer is not satisfied. He sent us a sample from our competitor, and it is much brighter than ours. Can you advise us on attaining a brighter, more burnished appearance in our process? J.K.
A. A burnished or high luster finish can have different definitions for different applications. Mass finishing can be used to obtain very low RMS finishes—under 2 RMS in some cases—as required for pre-plate decorative finishing, or for high-quality gears and other moving parts where silent operation and long life are desired. The parts in this application are powdered steel structural pieces, and they are not conducive to high luster finishing because of surface porosity. They can, however, get bright and shiny compared with those from a typical ceramic media deburring process.
A burnished finish (in mass finishing) is the result of pressure applied to the surface by the media to flatten out and polish the surface profile. The heavier and the smoother the media, the more effective the surface modification. If the initial surface was very rough, it may be necessary to use a cutting type media to lower the high spots. This leveling process can also be accomplished by using chemically accelerated processes. The more level the surface, the better the burnish. High pressure is obtained with heavy media, a deep mass and specific machine design and process settings. Ceramic and other manmade non-metallic burnishing media can weigh as much as 160 lbs/ft3. These can attain very good burnishing, but the ultimate is obtained with steel media that weighs from 270 to more than 300 lbs/ft3. Also, the compound plays a definite role; special compounds are used to obtain the shiniest finishes— more about that later.
All of the four equipment designs (tumble barrels, centrifugal barrels, centrifugal discs and vibrators) can produce good burnished finishes. Some do better than others, with centrifugal barrels being the best because they typically apply 15 to 20 times the force of gravity on the surface of the parts. So, if you are starting out and have to select equipment, and if the only consideration is the burnish finish, you will probably go with centrifugal barrels. These are not, however, the most practical choice when high volumes or automated processes are the objective.
In this case, with only two styles of vibratory finishers available, it’s best to choose the bowl machine because it is capable of using steel ballcone media. The ballcone is a good shape, because it is capable to reaching some areas that are missed by spherical media and it still gives a good burnish.
When burnishing in a vibratory finisher it is essential to set the weights for as smooth a rolling action as possible, with as deep a load as the machine can take. Some brands are better than others at achieving a good roll with heavy media. A complete discussion of weight settings is a topic of its own, but I advise you to increase the ratio of bottom to top weights, and to lower the lead angle. This will result in a smooth, gentle rolling action with very slow forward travel. The mass won’t travel up the incline to the internal separator, but you’ll need to hand separate the parts, anyway, to avoid part-on-part damage, which could occur on the screen separator.
If, after making these changes, the finish is still not equal to that obtained by the other supplier, this leads to a discussion about the role of compounds.
Good deburring compounds do not have much lubricity, and they generate very little foam. Their purpose is to keep the media clean and sharp, while cleaning and rinsing the parts and, perhaps, providing corrosion protection. They do not, therefore, make good burnishing compounds.
A burnishing compound should be very slippery to the feel. It should provide good cleaning so the part surfaces stay clean. Dirty parts won’t get a shiny finish. Also, some degree of foam is necessary to cushion the action between the contents of the mass. Without foam, the parts can damage each other, and even media can damage softer parts. Some foam has large bubbles, and some is more of a lather. For burnishing, the lather type of foam is best. It is good to try different foam styles and select one that gives the acceptable finish for you. Some of these compounds work well in flow through compounding mode while others require a closed system. Burnishing machines are notorious for splashing foam all over the area, and some operators put a cover over the machine.
Heavy foam must be removed from the parts before they dry. The usual method is it to rinse the parts after they leave the machine, perhaps applying a rust inhibitor, and then further buffing the parts in a cob dryer. It may be possible to rinse out the foam in the machine with a cleaner/inhibitor compound, and then perhaps go to a cob dryer. What you do will depend on the luster you are trying to attain. Some of the luster may be degraded if you rinse parts in the machine.
Another factor to consider is the time cycle. It takes longer to refine and burnish the surface than it does for most deburring. That 20 minute time cycle may stretch to an hour, or longer, before the surface gets really shiny. The unloading process may also take more time because small nicks and handling marks show up more on a finer finish.
You can see that good burnishing involves several steps and special media, special compound, special handling, and perhaps a special machine. It is good to carefully question the finish specification because providing a better burnish than is actually required can be very costly. In one application, a corporate officer insisted on jewelry-like burnishing for tools that were to be used by mechanics who most often would store them in a drawer full of other tools. Slight imperfections on the surface did not detract from the part’s function, and yet the officer insisted on final inspection under 4x magnification, and any nicking that could be detected would disqualify the part. The result was a non-profitable product line. So, be realistic about finishing requirements and do not take all the profit out of the production.
For those of you with other styles of mass finishing equipment, the same principals of high lubricity and foam quality hold true. Also, use high load levels—more than 50 percent full—in tumbling barrels and centrifugal barrels. High water levels may also be helpful. If you have variable speed, use lower rotation speeds to reduce part-on-part damage.
It has been shown that the inexpensive chemically accelerated vibratory surface finishing (CAVSF) process can reduce the average surface roughness.
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
Consider these five variables to determine what fits your application.